Technical Field
This disclosure relates to resonance frequency detectors and more particularly to a method and a circuit for determining resonance frequencies of a resonant device, and a method of filtering an output of a resonant device.
Description of the Related Art
Hard disk drive (HDD) applications use shock sensors (SS) in order to avoid unwanted write-read errors due to voice coil motor (VCM) harm caused by disk displacements that are due external causes (i.e. bumps, kicks, motor driving, mechanical stresses).
Unfortunately, these sensors have a mechanical resonance frequency that is close to their signal bandwidth and with amplitude that is several decibels above sensor sensitivity. Therefore, noise at this resonance frequency may be amplified and may even dangerously saturate amplification channels causing distortion of useful signals and long recovery times.
Different approaches are known in literature to prevent saturation caused by unwanted resonance peaking. They consist substantially in either:                low pass filtering the signals provided by the shock sensor before providing them to the respective amplification stages;        notch filtering the sensor signals to selectively attenuate certain frequency components.        
The first technique reduces the useful signal bandwidth, and the second technique may be correctly implemented only if the information about the resonance frequency is known.
The electrical equivalent circuit of a piezoelectric shock sensor is substantially a capacitor Cp connected electrically in parallel with a RLC series circuit (Cs-Rp-Ls), as shown in FIG. 1. The equivalent impedance is given by the following formula:
            Z      sensor        ⁡          (      jω      )        =                    1                  jω          ⁢                                          ⁢                      C            P                              ⁢              (                                            ω              2                        -                          ω              p              2                                                          ω              2                        -                          ω              s              2                                      )              =                  1                  jω          ⁢                                          ⁢                      C            P                              ⁢                        Z          ⁡                      (            jω            )                                    P          ⁡                      (            jω            )                              
wherein, for sake of clarity, zeros and poles of the impedance Zsensor are the zeroes of the polynomials Z(jω) and P(jω), respectively.
FIG. 2 shows the shock sensor (Cs, Cp, Rp, Ls) having first and second terminals IN, INP respectively coupled to inverting and non-inverting inputs, respectively, of a low noise amplifier 2 having an output OUT. A first parallel circuit 4 of a resistor RF and capacitor CF is coupled between the first terminal IN and the output OUT, and a second parallel circuit 6 of a resistor RF and capacitor CF is coupled between the second terminal INP and a reference terminal REF. In HDD applications, signals provided by the shock sensor are amplified by the low noise amplifier 2 in order to get noise immunity against physical noise (i.e., flicker and thermal noise) and noise related to external sources (i.e. coupled switching noise, RF interferences). For this reason, the zeroes of the polynomial Z(jω) cause resonance peaks in the AC response of the amplification stage:
            V      OUT        =                            2          ⁢                      C            p                                    C          F                    ⁢              (                                            P              ⁡                              (                jω                )                                                    Z              ⁡                              (                jω                )                                              +          1                )            ⁢              v        noise              ,
wherein vnoise is a noise voltage in input to the sensor.
For the above reasons, a reliable method of determining a resonance frequency of a device would be desirable.
The published US Patent Application No. 2010/0064809 discloses a system and a method for determining a mechanical resonance frequency of a sensor, consisting in applying a bias pulse signal to the sensor, detecting zero crosses of the voltage response of the sensor and determining the resonance frequency of the sensor in function of the detected zero-crosses.
Unfortunately, this prior method is not very accurate. Indeed, it is relatively difficult to determine with a high precision the instants in which the output voltage of the sensor nullifies, because of external disturbances (i.e., RF interferences, running clocks) and of the limited sensitivity of zero-cross detectors.